The present invention relates to a recorder having a semiconductor laser which can reproduce with densities of more than several tens of levels a picture such as a photograph having halftones.
A picture having halftones can be recorded by various techniques by which a laser beam is intensity-modulated. (1) In one of these techniques, an ultrasonic optical modulator is employed; (2) in another technique, the discharge current of a gas laser is changed, and (3) in a third technique, the current of a semiconductor laser is varied.
The first technique requires an expensive ultrasonic optical modulator and a fine adjustment device for setting the modulator to the Bragg angle. Thus, the first technique is disadvantageous in that the device has a high manufacturing cost and intricate construction. The gas laser discharge current modulation of the second technique suffers from drawbacks in that its modulation frequency range is very low, only several hundreds of Hertz, and the service life of the laser tube is reduced because the discharge current is varied. The third technique in which the current of the semiconductor laser is varied is also disadvantageous in that, since the semiconductor laser has an optical output current characteristic as indicated in FIG. 1, the optical output is greatly changed by changing the current only slightly, and accordingly it is considerably difficult to modulate the optical output by more than several tens of levels by changing the current.
Accordingly, an object of the invention is to provide a laser recorder which can record a picture having halftones with densities of more than several tens of levels. Another object of the invention is to provide a laser recorder which has several tens or several hundreds of optical modulation levels. A further object of the invention is to provide a laser recorder which can record a picture rich in halftones.